Despite being a valuable tool for volume measurement and the analysis
of complex geometry, the need for an external position sensor is
holding up the clinical exploitation of freehand three-dimensional
ultrasound. Some sensorless systems have been developed, using
speckle decorrelation for out-of-plane distance estimation, but their
accuracy is still not as good as that of sensor-based systems. Here,
we examine the widely held belief that accuracy can be improved by
limiting the distance measurements to patches of ultrasound data
containing fully developed speckle. Without speckle detection, we
observe that scan separation is systematically underestimated by 33.1%
in biological tissue. We describe a number of speckle detectors and
show that they reduce the underestimate to about 25%. We conclude
that speckle classification can improve the quality of distance
estimation, but not sufficiently to achieve accurate, metric
reconstruction of the insonified volume.

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